INTEGRATING FAULT-ROCK TYPES AND FABRICS WITH THERMO-KINEMATIC NUMERICAL MODELING TO UNDERSTAND THE MECHANICS OF A LOW-ANGLE NORMAL FAULT

During exhumation from the brittle-plastic transition to the surface of the Earth, exhumed fault zones develop complex overprints, which complicates their use as analogues to active crustal faults. Here we show that integrating regional thermo-kinematic modeling with fault-rock microstructure and composition can constrain the fault's geometric, thermal, frictional, and stress evolution.

We focus on the Boundary Canyon Detachment (NW of Death Valley, CA), which exposes a variety of fault rocks along its surface trace: quartz- & calc-mylonites, mixed cataclasites, and illite-rich gouges). Microstructural and geochemical observations on the fault rocks inform deformation conditions and composition, which are related to both temperature and friction by existing laboratory experiments. Detailed kinematic reconstruction and forward thermo-kinematic modeling, constrained by zircon (U-Th)/He cooling ages from the detachment footwall (Beyene, 2011), yield acceptable pre- to syn-exhumational fault geometries and geothermal gradients. These are used to map out the probable depth at which each fault rock-type was active, thereby constraining friction (static and rate-state) through depth and time. Using these results yields analytical static stress models and conceptual seismogenic potential of the fault.

Our geologically-constrained stress models support mechanical heterogeneity on the reconstructed detachment. At initiation, the fault likely crept at <40 MPa shear stress in the upper crust (<7 km depth), but slipped seismogenically at >40 MPa in the deeper brittle crust (>9 km depth). Seismogenic slip products in the deep brittle detachment are clearly linked to brittle reactivation of mylonitic C’-shear bands in older quartz-mylonites formed by plastic flow. Embrittlement localized consistently on rigid porphyroclasts, leading to formation of seismogenic cataclasites at both microscopic and hand-sample scales.